1. Field of the Invention
The present invention relates to a liquid crystal display device and a method for driving the liquid crystal display device. Further, the present invention relates to an electronic device provided with the liquid crystal display device in a display portion.
Note that in this specification, a semiconductor device means a circuit having a semiconductor element (e.g., a transistor or a diode) and a device having the circuit. The semiconductor device also means any device that can function by utilizing semiconductor characteristics. For example, an integrated circuit, a chip including an integrated circuit, a display device, a light-emitting device, a lighting device, and an electronic device are all semiconductor devices.
2. Description of the Related Art
As information society is developed, we gain information more often by information terminals, such as smartphones and personal computers, than by paper media. Thus, we keep watching screens from a short distance for a long period, and we overwork our eyes on a daily basis. There are multiple causes of eye strain, and one of them is a screen flicker.
In a display device, images are switched some dozen times per second. The number of image switching operations per second is called refresh rate. The refresh rate is also referred to as driving frequency. Such high-speed screen switching that cannot be recognized by human eyes is considered as a cause of eye strain. A typical example of a display means of an information terminal includes a liquid crystal display device (LCD). Thus, Non-Patent Documents 1 and 2 have proposed that the refresh rate of an LCD is lowered to reduce the number of image rewriting operations.
Driving methods (modes) of an active-matrix LCD are classified by a way of control of alignment of liquid crystal molecules. For example, a twisted nematic (TN) mode, a vertical alignment (VA) mode, an in-plane switching (IPS) mode, a fringe field switching (FES) mode, and the like are known. Pixel structures of LCDs are different among the driving methods.
In pixels of a TN or VA LCD, a pixel electrode is formed on one of a pair of substrates and a common electrode (also referred to as a counter electrode) is formed on the other substrate. An electric field that is perpendicular to the two substrate surfaces is formed between the pixel electrode and the common electrode to control alignment of liquid crystal molecules, so that the transmittance of the pixels is controlled.
In contrast, in an IPS or FFS LCD, a common electrode is formed on the same surface where the pixel electrode is formed. In an IPS LCD, the common electrode and the pixel electrode have a comb-shape pattern and are formed on the same insulating film. An FFS mode is an improved display mode of the IPS mode, and in the FFS LCD, a pixel electrode and a common electrode are formed to face each other with an insulating film therebetween. For example, the pixel electrode in an FFS LCD has a structure in which a plurality of slits are provided, and alignment of liquid crystal molecules is controlled by an electric field (fringe field) formed between a fringe of a pixel electrode and a common electrode; thus, an FFS LCD has a wider viewing angle and higher transmittance than an IPS LCD.
Various improvements have been made on an FFS LCD, and for example, Patent Document 1 discloses that a second common electrode is formed on a substrate on which a pixel electrode is not formed, and the LCD can have a high-speed response and a wide viewing angle by a potential applied to the second common electrode.